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COPENHAGEN—At some point in the next year, nations planning to join climate talks are going to propose greenhouse gas caps. How will the international negotiators decide how much of the pollution-cutting burden each country must shoulder?

One solution, first proposed in Kyoto, Japan, in 1997, is to have each country simply volunteer what it can and then control emissions within its borders by a cap-and-trade system or by a variety of other means. But back then, this approach led to uneven emissions responsibilities between countries: Denmark, for example, had huge responsibilities to cut its emissions—an 8% cut relative to 1990—while Russia got to keep its emissions flat. And China and India weren't asked to cut anything at all. There's a variety of reasons that the system is unfair, but here's one: Poor people in European countries with carbon caps have to pay for them, generally through higher energy prices, while rich people who drive SUVs in China don't.

Today, an international team of scientists proposed a new way of deciding who needs to cut what: Set up a scheme in which every person on Earth has the same climate pollution limit— "a global personal emissions cap."

The system would work by first setting a global limit for each year—ideally to be determined by the science, said Heleen de Coninck of the Energy Research Centre of the Netherlands. If, for example, negotiators set the global emissions limit for 2030 at 30 billion tons of carbon dioxide emitted, each nation would determine what its citizens' individual responsibilities to cut emissions were to achieve that cut and then add them up.

Individual Americans in 2005, for example, were responsible for roughly 19 tons of CO2 each per year on average. China's average was about 4 tons. Under the scientists' proposal, the world limit in 2030 would be 11 tons per person. So negotiators would calculate the per capita emissions cuts that each country would be responsible for. It would be up to individual governments to decide how to divvy up the cuts, but the end result would be that the average U.S. emitter would have to cut more than the average Chinese.

According to scientists, roughly 275 million people in the United States would be above the cap. Ironically, that's the same number of people in China who would be above the limit—though the Chinese emitters would, for the most part, have far less personal emissions to cut. De Coninck says that scenario would undercut a Chinese argument, often heard at negotiations, that they have lots of poor people who shouldn't have to suffer under an arduous emissions limit. "You have to treat every individual in the world the same, regardless of what country they're in," says de Coninick.

There are a number of problems with the system, she admits. First, the plan only works with energy-related carbon dioxide—trying to add other emissions makes it almost impossible to sort out. In addition, the system doesn't account for deforestation or emissions connected to agriculture, which, as the sessions at this conference are making clear, is a huge part of the world greenhouse gas emissions. And the system says nothing about how nations will divvy up the emissions cuts once they're assigned to each country. For example, in a country that doesn't emit much greenhouse gases, there's nothing stopping a dictator from forcing the cuts caused by emissions caps on everyone, including poor people who don't emit anything.

The scientists proposing the approach include researchers from Princeton University, Harvard University, Energy Research Centre of the Netherlands, and the Fondazione Eni Enrico Mattei.

(This article has been corrected with the words "energy related" added to second-to-last paragraph.)